add Carousel widget and WrapGrid::centre_last_row

Forge's app switcher needs two layouts the existing widget set didn't cover. The desktop grid wants a partial last row centred under the rows above (3 tiles → row 1: two, row 2: one centred) so a 7-of-9 leftover band reads balanced rather than left-aligned. The mobile variant wants a horizontal carousel where the focused tile sits centred in the viewport at a configurable fraction of its width and its neighbours peek out on the sides at a fixed gap.
Extend `WrapGrid` with `centre_last_row( bool )`. When set, layout offsets a row that has fewer than `columns` children by `(missing * (cell_w + spacing)) / 2` so it stays centred inside the content rect. Defaults to false; every existing call site continues to land tiles flush-left. Covered by three layout tests (centred partial row, full row no-op, off-by-default).
Add the `Carousel` widget at `src/widget/carousel/`. It is a pure layout primitive: `focused_width_frac` (0.05–1.0, clamped), `gap` and `offset` are owned by the caller, leaving drag / inertia / snap policy to the host so the compositor can plug in its existing touch pipeline. Each child gets a rect at `base_x + idx * (child_w + gap)` and the full viewport height; `snap_offset( viewport_w, idx )` translates index to centring offset and `focused_index( viewport_w )` rounds the current offset back to the nearest tile. Plumbed into `Element::Carousel` with the matching arms in `widget/element.rs` and walker in `draw/layout.rs`; re-exported as `ltk::{ Carousel, carousel }`. Covered by nine unit tests (layout, offset shift, snap / focus round-trip, frac clamp, child height) plus a `cargo run --example carousel` demo with Prev / Next / arrow-key navigation against an external offset state. The example is wired into the `examples` Makefile target.
Updates the widget catalogue and the `widget/mod.rs` landing comment to list the carousel under "Clipping wrappers" and to mention `centre_last_row` in the grid section.

docs/widgets.md

@@ -22,7 +22,7 @@ patterns built from these widgets, see [`docs/cookbook.md`](./cookbook.md).
 - [Decoration and chrome](#decoration-and-chrome)
   - [`container`](#container) · [`separator`](#separator) · [`img_widget`](#img_widget)
 - [Clipping wrappers](#clipping-wrappers)
-  - [`scroll`](#scroll) · [`viewport`](#viewport) · [`flex`](#flex)
+  - [`scroll`](#scroll) · [`viewport`](#viewport) · [`flex`](#flex) · [`carousel`](#carousel)
 - [Overlays and feedback](#overlays-and-feedback)
   - [`spinner`](#spinner) · [`toast`](#toast) · [`tooltip`](#tooltip) · [`combo`](#combo) · [`tabs`](#tabs) · [`notebook`](#notebook) · [`dialog`](#dialog)
 - [Pickers](#pickers)
@@ -522,6 +522,47 @@ spacer siblings.
 **See also**: [`spacer`](#spacer) for invisible fillers,
 [`column`](#column) and [`row`](#row).
 
+### `carousel`
+
+Horizontal carousel: the focused child sits centred in the viewport at
+`focused_width_frac` of the viewport width and its neighbours peek out
+on the left / right at `gap` separation. The widget is a pure layout
+primitive — the `offset` (positive shifts content right) is owned by
+the caller, so drag / inertia / snap-ease live in the host (compositor,
+gesture recogniser, etc.).
+
+**When**: mobile-style app switchers, image / story strips, any
+single-focus horizontal navigation where neighbours hint at the next /
+previous tile.
+
+```rust,no_run
+# use ltk::{ button, carousel, container, Element, Color, Corners };
+# #[ derive( Clone ) ] enum Msg { Open( usize ) }
+# fn _ex( offset: f32 ) -> Element<Msg> {
+carousel()
+    .focused_width_frac( 0.8 )
+    .gap( 16.0 )
+    .offset( offset )
+    .push( container( button::<Msg>( "Tile 1" ).on_press( Msg::Open( 0 ) ) )
+        .background( Color::rgb( 0.95, 0.4, 0.4 ) )
+        .radius( Corners::all( 12.0 ) ) )
+    .push( container( button::<Msg>( "Tile 2" ).on_press( Msg::Open( 1 ) ) )
+        .background( Color::rgb( 0.95, 0.85, 0.3 ) )
+        .radius( Corners::all( 12.0 ) ) )
+.into()
+# }
+```
+
+Helpers on the widget translate between offsets and indices:
+`snap_offset( viewport_w, idx )` returns the offset that centres tile
+`idx`; `focused_index( viewport_w )` rounds the current offset to the
+nearest tile. The runnable demo at
+[`examples/carousel.rs`](../examples/carousel.rs) shows Prev / Next
+buttons and arrow-key navigation against an external `offset` state.
+
+**See also**: [`scroll`](#scroll) for free vertical / horizontal panning
+of arbitrary content; [`tabs`](#tabs) for a non-touch alternative.
+
 ---
 
 ## Overlays and feedback
@@ -834,6 +875,11 @@ grid( 4 )
 
 Wrap inside [`scroll`](#scroll) when the grid may overflow.
 
+`centre_last_row( true )` shifts a partial last row so its tiles sit
+centred under the full rows above instead of left-aligned — useful for
+app switchers and gallery layouts where a 7-of-9 leftover band reads
+better balanced.
+
 ### `spacer`
 
 An invisible flexible filler. Inside a column / row, absorbs leftover